The present invention relates to a hydraulic control system for a hydraulic excavator, and more particularly to a hydraulic control system for a hydraulic excavator which allows operating power and operating speed to be improved as necessary.
Generally, a hydraulic excavator comprises a lower traveling body and a revolving superstructure. The revolving superstructure has an operating machine provided with a boom, an arm, a bucket, and the like. The traveling apparatus, the revolving apparatus, the operating machine, and other apparatus used in a hydraulic excavator are operated by hydraulic actuators that are separately provided therein. In other words, various hydraulic circuits are mounted on a hydraulic excavator. Generally, such hydraulic circuits comprise a main circuit and a pilot circuit. The main circuit includes a hydraulic actuator, a flow-rate control valve, a hydraulic control valve, a direction changeover valve, a servo valve, and other hydraulic devices. The pilot circuit is adapted to provide instructions to the flow-rate control valve, the hydraulic control valve, the direction changeover valve, the servo valve, etc. so that they operate as required. As a pilot system this pilot circuit comprises hydraulic pressure, pneumatic pressure, electrical signals, means for combining them, and other means. Accordingly, so-called hydraulic control circuits generally represent the flow-rate control valve, hydraulic control valve, direction changeover valve, servo valve, etc. of the main circuit, as well as pilot circuits related to them. These pilot circuits control the amount of oil supplied to the hydraulic actuator of the main circuit and the oil pressure thereof.
It has in recent years become the practice to control such a hydraulic control circuit of a hydraulic excavator in such a manner that the hydraulic horsepower is constantly set at a fixed level (hereafter, this control will be referred to as power constant control). This power constant control is conducted with a view to causing the hydraulic horsepower to coincide with the engine output to as practical an extent as possible. By virtue of this control, overall output losses can be reduced. A more advanced type of hydraulic control circuit is generally so arranged as to limit the power constant control when the pressure of the main circuit approaches relief pressure (hereafter, this control will be referred to as cut-off control). Incidentally, the aforementioned relief pressure refers to the maximum hydraulic pressure of the main circuit. When the actuator is subjected to a heavy load or the like, the hydraulic pressure of the main circuit rises, and the relief pressure is provided to limit the extent of this rise in pressure so as to protect the circuit and its component devices from becoming damaged by the hydraulic pressure. This relief pressure is set by the hydraulic control valve (hereafter referred to as the relief valve). Returning to the cut-off control, this control is also designed to reduce output losses. More specifically, when the pressure of the main circuit approaches the relief pressure, the flow rate decreases on the basis of the power constant control. Since the flow rate is still high, this cut-off control is effected to further reduce the flow rate sharply. If this cut-off control is not provided, a large amount of oil would return to the oil sump when the circuit pressure is close to the relief pressure. At this time, output loss would occur due to the rise in oil temperature and the occurrence of relief noise.
Referring now to FIGS. 1 to 3 which illustrate an example of a conventional hydraulic control apparatus for a hydraulic excavator having the above-described arrangement, a detailed explanation will be given of the hydraulic control apparatus. The hydraulic circuit shown in FIG. 1 is an example of a generally adopted hydraulic circuit of this type. It goes without saying that this circuit is provided with a power constant control valve 30 and a cut-off control valve 10. In addition, this hydraulic circuit is composed of main circuits P and pilot circuits Pc. The main circuit P (the relevant circuits and the associated hydraulic pressure levels are denoted by the same reference character) includes a hydraulic tank 35, a variable capacity-type hydraulic pump 40, a changeover valve 41, various actuators 42n, a relief valve 60, and circuits connecting them.
A description will now be given of the flow of oil. Oil from the hydraulic tank 35 is supplied to the changeover valve 41 via the variable capacity-type pump 40. Here, the oil is either returned to the tank 35 or supplied to the actuators 42n so as to actuate the same. As described above, the relief valve 60 limits the relief pressure of the main circuit.
The pilot circuit Pc comprises a constant capacity-type hydraulic pump 50, and a servo valve 20, a cut-off control valve 10, a power constant control valve 30, which constitute a hydraulic control system, as well as circuits P1, P2, P3, Pc1, Pc2, Pc3, Pc4, and Pc5 which connect them.
A description will now be given of the relationships between the pilot circuit and the hydraulic control system. The pilot pressure Pc5 is supplied to the servo valve 20. If the pilot pressure Pc5 is large, the servo valve 20 controls the pilot pressure Pc2 in the direction in which the amount of oil discharged by the variable capacity-type hydraulic pump 40 increases. If the pilot pressure Pc5 is small, the servo valve 20 controls that pressure in the direction in which said amount of oil discharged decreases. This pilot pressure Pc2 acts on the variable capacity-type hydraulic pump 40 and controls the amount of oil discharged thereof, in the above-described manner.
A description will now be described of the power constant control valve 30 and the cut-off control valve 10. Upon receipt of the pilot pressure P3 from the main circuit P, the power constant control valve 30 controls the pilot pressure Pc4 and effects control in such a manner that the hydraulic horsepower remains constant (hydraulic pressure P .times.flow rate Q=constant) (the results of this power constant control will be hereafter referred to as power constant characteristic C), as shown in FIG. 2. Meanwhile, upon receipt of the pilot pressure Pc4, the cut-off control valve 10 outputs the pilot pressure Pc5. In addition, the pilot pressure P2 from the main circuit is also input to the cut-off control valve 10. Normally (when the main circuit is not set under the relief pressure), the pilot pressure Pc4 (one in which the pilot pressure Pc from the hydraulic pump 50 has been controlled through the circuits Pc1, Pc3, and the power constant control valve 30) is input to the cut-off control valve 10, which then outputs the pilot pressure Pc5 (the pressure being Pc4 Pc5) to the servo valve 20. However, when the hydraulic pressure P of the main circuit P approaches the relief pressure, the pilot pressure P2 (the pressure being P2 P), in cooperation with the pilot pressure Pc5 which is the self output pressure of the cut-off control valve 10, overcomes the force of the spring urged in the direction in which the cut-off control valve 10 is opened, thereby closing the cut-off control valve 10. The pilot pressure Pc4 is shut off through this operation. Consequently, the above-described power constant control is cut off. In other words, the power constant characteristics are canceled in the vicinity of the relief hydraulic pressure, as shown in FIG. 2. Hence, a cut-off characteristic B is obtained.
Since these cut-off characteristics B are essential to the description of the present invention, a specific arrangement of the cut-off control valve will be described on the basis of an example shown in FIGS. 3 and 3A. When the pilot pressure P2 from the main circuit is below the relief pressure, a spool 12 is pressed downward, as viewed in the drawing, by a spring 11. For this reason, the pilot pressure Pc4 is output as the pilot pressure Pc5. However, when the pilot pressure P2 from the main circuit approaches the relief pressure, the pilot pressure P2, in cooperation with the pilot pressure Pc5 which is the self output pressure of the cut-off control valve 10, overcomes the urging force of the spring 11, and thus pushes the spool 12 upwardly is viewed in the drawing, thereby gradually shutting off the output pilot pressure Pc5 through a notch 13 of the spool 12. It should be noted that the cut-off characteristic B has a slight inclination in FIG. 2 which is attributable to the effect of the notch 13 and the spring 11.
However, even with the hydraulic control system for a hydraulic excavator which has been well devised, as described above, in the case of an operation in a hydraulic region where the cut-off characteristic B can function readily (i.e., the region of a heavy load in which the relief pressure is liable to occur), the amount of oil declines immediately to a minimum amount with the slightest increase in hydraulic pressure, as can be seen from FIG. 2. In consequence, there is a drawback in that the speed of the actuator declines sharply. Furthermore, under the relief pressure, the operation of the actuator stops. Accordingly, in such a region of a head load, even if the operator desires to increase some more power and speed, the operator's desire cannot be attained. Hence, even with the hydraulic excavator which has thus been contrived well, the operator may disadvantageously determine that such a hydraulic excavator is a machine having a poor operating performance.